The groups say the two Democratic senators are going too far in a bid to win Republican support for gun control legislation expected to hit the Senate floor next week.

“Everyone supports background checks except for NRA leadership,” said Josh Horwitz, executive director of the Coalition to Stop Gun Violence, one of the advocates alarmed by the changes Schumer and Manchin are suggesting.

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“I think we should be tough on this deal,” he said. “If the vote fails and a legislator votes down background checks, there’s hell down to pay.”

At issue is whether to exempt private gun sales between friends and acquaintances from record-keeping requirements that now only cover licensed gun dealers.

Shortly before the Easter recess, Schumer and Manchin proposed that such transactions be subject to background checks but exempt from the record-keeping rules.

He added that exempting record-keeping rules on sales between friends “falls short” because “criminals get their guns from their friends”.

Brian Malte, director of mobilization at the Brady Campaign to Prevent Gun Violence, also said gun sales between acquaintances should not be exempted from the requirement.

“The record is important because if you ask law enforcement, the vast majority will say when we want to trace a crime gun we need to have access to a paper record,” he said. “How do you know someone you just met doesn’t have domestic-violence restraining order against them? How much do you know about an acquaintance, and how do you define what an acquaintance is?”

The White House has signaled that Obama is eager to sign a gun-violence package into law, even one diluted by concessions to Republicans and centrist Democrats.

Dan Pfeiffer, a senior adviser to the president, said at a breakfast sponsored by Politico Wednesday, that Obama would not hold out for a perfect bill.

“What the president wants to sign is the strongest gun bill he can sign,” he said. “What we have to make sure is that whatever we do is better than current law.”

A spokesman for Schumer did not respond to a request for comment.

Manchin has said he is “talking to everybody” in a bid to build bipartisan support for expanded background checks, but he has declined to discuss the details of his negotiations.

Gun control advocates claim only the leadership of the NRA opposes expanded background checks and argue Schumer and Manchin should not concede ground when public opinion is overwhelmingly on their side.

“My personal opinion is that expanding record-keeping is not an issue for me and I don’t think it is for most gun owners,” said Scott Crider, founder of the United Network of Rational Americans, an advocacy group for gun owners who support universal background checks.

Coburn has suggested that expanded background checks could be enforced through sting operations. He argues that law abiding gun owners can be trusted to perform background checks because they do not want to sell firearms that could be used in crimes.

But gun control groups say it will be very difficult to enforce expanded background checks if they are not accompanied by records.

“Dealer record-keeping is a critical part of making universal checks enforceable. We've had dealer record-keeping on background checks — the ones on the guns the dealers sell directly — for 20 years. It ensures the checks are being done and makes it possible to trace a gun recovered in a crime," said Arkadi Gerney, a gun violence prevention expert at the Center for American Progress.

Mark Glaze, director of Mayors Against Illegal Guns, said the modified Schumer-Manchin proposal would be a step in the right direction because many guns used in crimes are bought from unlicensed sellers at gun shows and through Internet venues such as ArmsList.com.

“The bulk of the criminal activity that we’re trying to capture takes place in the commercial marketplace,” he said.

But Glaze acknowledged eliminating the record-keeping requirement for many private sales would make it more difficult to enforce background checks.

“Very often criminals will claim that they got their guns from friends. Very often they did in fact get their guns from friends. You don’t want that to become a universal excuse,” he said.